Transmission gratings with a period of 100 nm for extreme ultraviolet interference lithography are
fabricated with 2 groups of 50 nm thick Cr bars on a 100 nm thick Si3N4 film. The fabrication process
starts with depositing Si3N4 on both sides of (100) Si wafers by LPCVD, followed by electron beam
lithography of ZEP520A resist, evaporation of Cr and resist lift-off. A 120 nm thick stop layer of Au is
then evaporated onto the surrounding area to eliminate unwanted transmission. Finally, a pair of Si3N4
windows are opened on the back side by dry etching, and the Si under the grating pattern is removed by
KOH anisotropic wet etching. Diffraction measurement shows an acceptable first order efficiency of
the gratings at the wavelength of 13.4 nm. Using the fabricated gratings at the interference lithography
beam line of Shanghai Synchrotron Radiation Facility, economic and efficient fabrication of gratings
with a doubled pitch, namely 50 nm period gratings, can be expected.
X-ray transmission gratings (TG) have attracted much interest because of its wide use in x-ray
telescope, synchrotron radiation facilities, and target diagnostic in inertial confinement fusion, etc. In
this work, a 200 nm period master TG to diffract x-ray in the energy range 0.1-8keV has been
successfully fabricated by electron beam lithography followed by gold electroplating. In fabrication
processes, 500 nm resist was exposed by focused electron beam on polyimide free-standing-membrane
coated with a Cr/Au plating base. According to numerical simulation, the proximity effect due to
electron back-scattering from the substrate can be sharply reduced because of the thin polyimide
free-standing membrane substrates. PMMA resist was chosen due to its high resolution and good
performance in subsequent processes. After delicate dose test and shape modification of the proximity
effect caused by electron front-scattering, resist grating bars with 95 nm width and 200 nm period were
achieved. Subsequently, resist patterns were transferred to gold layer by electroplating. In future work,
with this master mask of TG, thousands of TG to diffract x-ray can be sufficiently replicated using
x-ray lithography.
Grating patterns with approximately 150 nm period were achieved by X-ray lithography with a
single exposure through a 300 nm period grating mask, which was manufactured by e-beam
lithography. BPM simulation of the X-ray propagation through the mask structure, which acts here
in a way much like that of a wave guide with many layers, was carried out. Considering also the
light propagation in the uniform space between the mask and the wafer, preferable parameters of
the optical setup, such as the exposure dose, the distance between the mask and the wafer, and
resist thickness, are suggested and their process windows are discussed. The dependence of the
resulted pattern profiles on the mask design is analyzed and an optimized design of the mask
grating is presented for this process. By carefully choosing the process parameters, the doubling of
grating resolution by X-ray lithography can be expected under precise control.
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